The invention relates to a capacitor, in particular a multilayer capacitor with internal electrodes of base metals, said capacitor comprising a ceramic dielectric material and at least two electrodes, said dielectric material essentially consisting of a dielectric composition with a doped barium-calcium-zirconium-titanate.
Ceramic multilayer capacitors are customarily manufactured by alternately stacking layers for the dielectric material, which are made of a green, dielectric ceramic composition, and layers for the electrodes, which are made of a metal paste, whereafter the stacks of ceramic and metal layers are co-sintered.
The quality of multilayer capacitors is governed by the chemical composition of the material used for the dielectric and the electrodes, as well as by the manufacturing conditions. As regards the manufacturing conditions, in particular, the sintering conditions play a role. Depending on the sinter atmosphere, opposed oxidation and reduction reactions may take place. During sintering in a reducing atmosphere, barium titanate and the derivatives thereof, such as the doped barium-calcium-zirconium-titanates, become semiconducting; in this condition, they are unsuitable as a dielectric material. Multilayer capacitors can only be sintered under oxidizing conditions if the electrode material is composed of rhodium, palladium or platinum. However, rhodium and platinum are very expensive, up to 50% of the manufacturing costs can be attributed to the use of these materials. Therefore, there is a tendency to use the much cheaper nickel or the alloys thereof instead of rhodium and platinum. However, nickel oxidizes if it is sintered under oxidizing conditions, so that multilayer capacitors having nickel electrodes must be sintered in an inert or slightly reducing atmosphere. Sintering in a reducing atmosphere would cause the tetravalent titanium in the barium titanate to be reduced to trivalent titanium, which would lead to an extreme reduction of the insulation resistance of the capacitors. Meanwhile, however, it has been achieved to lessen the reducibility of the barium titanate by doping it with additives, which are acceptors such as Cr.sub.2 O.sub.3, Co.sub.2 O.sub.3 or MnO.
However, these dopants cannot prevent the formation of oxygen vacancies in the crystal during sintering in a reducing atmosphere, which oxygen vacancies drastically reduce the service life of the capacitors. Said oxygen vacancies have a high mobility in the crystal lattice and migrate under the influence of an electric voltage and temperature. As a result, the insulation resistance decreases with time.
The formation of oxygen vacancies can be partly undone by subjecting the capacitors, after they have been sintered in a reducing atmosphere, to a tempering process in a slightly oxidizing atmosphere at temperatures between 600.degree. C. and 1100.degree. C. In this process, the oxygen vacancies in the lattice are filled again. A disadvantage of said tempering process is the substantial decrease of the dielectric constant .di-elect cons. and the negative influence on the .DELTA.C/.DELTA.t-curve, i.e. on the temperature-dependence of the dielectric constant.
In order to overcome said complex difficulties which are encountered in the manufacture of ceramic capacitors with base-metal electrodes, it is proposed, in U.S. Pat. No. 5,319,517, to use a multilayer ceramic chip capacitor comprising internal electrodes and dielectric layers, the dielectric material of which contains a dielectric oxide having a composition of the following formula: EQU (Ba.sub.1-x-y Ca.sub.x Sr.sub.y)O!.sub.m (Ti.sub.1-x Zr.sub.x)O.sub.2,
wherein 0.ltoreq.x.ltoreq.0.25, 0.ltoreq.y.ltoreq.0.05, 0.1.ltoreq.z.ltoreq.0.3, and 1.000.ltoreq.m.ltoreq.1.020 and having added thereto a manganese oxide and/or a compound which converts to the oxide upon firing in an amount of 0.01 to 0.5% by weight calculated as oxide (MnO), an yttrium oxide and/or a compound which converts to the oxide upon firing in a quantity of 0.05 to 0.5% by weight calculated as oxide (Y.sub.2 O.sub.3), a vanadium oxide and/or a compound which converts to the oxide upon firing in an amount of 0.005 to 0.3% by weight calculated as oxide (V.sub.2 O.sub.5), a tungsten oxide and/or a compound which converts to the oxide upon firing in a quantity of 0.005 to 0.3% by weight calculated as oxide (WO.sub.3), and nickel or a nickel alloy being used as the material for the inner electrodes of said multilayer capacitor. The ever increasing requirements in terms of service life and reliability can hitherto not be met by these multilayer capacitors.